Recently, as a new memory device, a semiconductor memory device called Resistive Random Access Memory (RRAM) is noted. The RRAM uses a resistance memory element which has a plurality of resistance states of different resistance values, which are changed by electric stimulations applied from the outside and whose high resistance state and low resistance state are corresponded to, e.g., information “0” and “1” to be used as a memory element. The RRAM highly potentially has high speed, large capacities, low electric power consumption, etc. and is considered prospective.
The resistance memory element has a resistance memory material whose resistance states are changed by the application of voltages sandwiched between a pair of electrodes. As the typical resistance memory material, oxide materials containing transition metals are known.
The related arts are disclosed in, e.g., Japanese published unexamined patent application No. 2003-008105, Japanese published unexamined patent application No. 2004-301548, Japanese published unexamined patent application No. 2005-039228, and S. Q. Liu (“Electrical-pulse-induced reversible resistance change effect in magnetoresistive film”, Appl. Phys. Lett., vol. 76, p. 2749, 2000).
RRAM uses the resistance memory element whose high resistance state and low resistance state are reversibly changed by application of voltages, but its operational mechanism has not be cleared. The inventors of the present application have an idea as one operational mechanism of the resistance memory element that the filament-shaped property changed (current path) formed in the resistance memory material would contribute.
This filament-shaped current path would be formed in a part where an electric field is locally concentrated, and the structure of the conventional resistance memory element, which is similar to the parallel plate capacitor, has found difficult to control the position and the density of the filament-shaped current path. This would be a barrier to further improving the density.